There exist different approaches for synthesizing polymers from vegetable oils. The first, the most widespread consists of considering triglycerides as base materials, the latter being able to be epoxidized and then for example alcoholized or hydro formulated, in order to make them functionable and polymerizable.
An oil is a mixture of triglycerides (triesters) formed by condensation of fatty acids and of glycerol. The high number of types of fatty acids (up to 24) present in each fat and the multiple possibilities of their combinations with glycerol molecules ensure that fats are highly complex mixtures of compounds, the properties of which vary from one oil to another. The nature of the triglycerides may therefore vary within a same oil.
The reactive sites present in a triglyceride are mainly double bonds and ester functions. The reactivity of double bonds allows the introduction of hydroxyl functions, thereby allowing access to multihydroxlated monomers. It is nevertheless impossible to obtain triglycerides having perfectly defined structures and functionalities.
The synthesis of polyols from vegetable oil is well described in the literature since the latter are excellent precursors for synthesizing polymers. These materials have one popularity because of the natural origin of the precursors and of the attractive properties provided by the structure and the composition of the vegetable oils. The reactive sites in all fats are ester functions and double bonds. Certain oils also have other groups such as hydroxyls or epoxides.
The double bonds of these compounds are generally not sufficiently reactive for being used as sites of radical polymerization. Nevertheless, at a high temperature (330° C.), the double bonds may migrate along the backbone in order to form conjugate sites, which facilitates condensations of the Diels-Alder type. Oligomers were synthesized by vulcanization of oils with sulfur monochloride and used as additives in the gum industry for example. Oligomers were also synthesized by cationic polymerization in the presence of boron trifluoride (Croston et al., J. Amer. Oil Chem. Soc. 1952, 331-333), for application in the formulations of inks. Other reactions involving double bonds, such as polymerization by metathesis gave the possibility of obtaining oligomers (Refvik et al., J. Amer. Oil Chem. Soc. 1999, 76, 93-98) and the materials which stem from this are rarely utilizable because they are very poorly defined.
It is therefore necessary to better control functionalization of the vegetable oils.
As already indicated, the presence of double bonds on the backbone allows introduction of hydroxyl groups. The latter may be achieved by direct oxidation of the double bonds, which consists of having an oxygen screen pass through the oil heated to 135° C. (G., Soucek et al. “Spectroscopic investigation of blowing process of soybean oil”, Surface Coatings International, Part B, Coatings Transactions. 2003, 86: 221-229). Control of the oxidation is not satisfactory and many byproducts are formed such as peroxides, aldehydes, ketones, splittings of chains, etc. The only advantage of these polyols is their low price cost and their synthesis is achieved in a single step, in spite of the many treatments applied to the final product (odors, high acid index, dark color, etc.).
An organometal catalyst may also be used in order to better control the oxidation reaction (WO2006/094227; WO2007/143135) in the presence of an oxidant.
Polyols having primary hydroxyls may be prepared by hydro formulation of the unsaturations (Guo et al., J. of Polymers and the Environment. 2002, 10: 49-52). This method involves a reaction between carbon monoxide and dihydrogen, causing the formation of an aldehyde group which is converted into a hydroxyl by hydrogenation. Rhodium-based catalysts generally used are very efficient (conversions close to 100%) but also very costly. Conversely, cobalt-based catalysts are inexpensive but less efficient. By ozonolysis of the double bonds, it is also possible to obtain polyols having terminal hydroxyl groups (Guo et al., J. of Polymer Sci., 2000, 38: 3900-3910). The ozone passes through a solution of vegetable oil and ethylene glycol, in the presence of an alkaline catalyst.
Another route for accessing polyols consists of conducting a preliminary reaction of epoxidation of the unsaturations. Many studies described in the literature describe the epoxidation of fats (Swern, et al. J. Am. Chem. Soc. 1944, 66, 1925-1927; Findley et al., J. Am. Chem. Soc. 1945, 67, 412-414: U.S. Pat. No. 5,026,881; U.S. Pat. No. 3,328,430; Petrović et al., Eur. J. Lipid Sci. Technol. 2002, 104: 293-299 and U.S. Pat. No. 4,647,678). Petrovic recently demonstrated the possibility of achieving epoxidation of vegetable oil via an enzymatic route (Vic{hacek over ( )}ek, T. et al., J. Amer. Oil Chem. Soc. 2006, 83: 247-252) or catalyzed with an ion exchange resin (Sinadinović-Fis{hacek over ( )}er et al., J. Amer. Oil Chem. Soc. 2001, 78: 725). Nevertheless the most common route is the use of a peracid formed in situ, generally hydrogen peroxide in the presence of a carboxylic acid (most often formic acid in a catalytic amount). The reaction is conducted between 50 and 80° C. for 1 to 4 hours.
It is important to emphasize that the epoxidation of fats already having a primary alcohol at the end of the chain was never achieved from a peracid formed in situ. This reaction cannot be conducted under the same conditions as previously because of secondary reactions between the carboxylic acid and the terminal alcohol.
The epoxidized vegetable oils were used as intermediates in many syntheses. The Petrovic group describes the opening of the epoxides with alcohols, inorganic acids and by hydrogenation under acid catalyses (Guo et al., J. Polym. Sci. Part A: Polym. Chem. 2000, 38: 3900-3910). Epoxidized triglycerides were modified by reaction with HCl or HBr in the presence of acetone (solvent) by hydrogenation with H2 in the presence of isopropanol and of Raney nickel as a catalyst, and finally by methanol in the presence of isopropanol and of an acid catalyst (for example fluoboric acid).
These functional triglycerides have different reactivities: the one derived from the opening with methanol being the most reactive towards isocyanates for the chemistry of polyurethanes. Petrovic increases the reactivity of the polyols obtained by ethoxylation (opening of ethylene oxide by the secondary alcohol under acid catalyses), which converts the secondary alcohols into primary alcohols. However it is noted that the use of an acid catalyst causes secondary reactions, such as the formation of a methyl ester during the opening of the epoxide with methanol. The solution lies in the use of a specific catalyst for opening the epoxide and operating at lower temperatures in order to avoid secondary coupling reactions. Finally, it is interesting to note that the US Patent Application, published under number 2006/7045577, describes the synthesis of polyurethane from soya bean oil according to a two-step process: (i) the oil is epoxidized from conventional methods using a peracid and (ii) the epoxidized soya bean oil reacts with carbon dioxide in order to give a carbonated vegetable oil. The reaction of this product with a diamine allows access to a polyurethane without using any isocyanates.